Colorectal carcinoma (CRC) is the second most frequent cancer and the second leading cause of cancer-associated deaths in the U.S. and Western Europe. The overall five-year survival rate for patients has not meaningfully improved in the last three decades. Prognosis for the CRC cancer patient is associated with the depth of tumor penetration into the bowel wall, the presence of regional lymph node involvement and, most importantly, the presence of distant metastases. The liver is the most common site for distant metastasis and, in approximately 30% of patients, the sole initial site of tumor recurrence after successful resection of the primary colon cancer. Hepatic metastases are the most common cause of death in the CRC cancer patient.
The treatment of choice for the majority of patients with hepatic CRC metastasis is systemic or regional chemotherapy using 5-fluorouracil (5-FU) alone or in combination with other agents such as leviamasole. However, despite extensive effort, there is still no satisfactory treatment for hepatic CRC metastasis. Systemic single- and combination-agent chemotherapy and radiation are relatively ineffective emphasizing the need for new approaches and therapies for the treatment of the diseases.
A gene therapy approach is being developed for primary and metastatic liver tumors that exploits the transcriptional differences between normal and metastatic cells. This approach involves linking the transcriptional regulatory sequences (TRS) of a tumor associated marker gene to the encoding sequence of an enzyme, typically a non-mammalian enzyme, to create an artificial chimaeric gene that is selectively expressed in cancer cells. The enzyme should be capable of converting a non-toxic prodrug into a cytotoxic or cytostatic drug thereby allowing for selective elimination of metastatic cells.
The principle of this approach has been demonstrated using an alpha-fetoprotein/Varicella Zoster virus thymidine kinase chimaera to target hepatocellular carcinoma with the enzyme metabolically activating the non-toxic prodrug 6-methoxypurine arabinonucleoside ultimately leading to formation of the cytoxic anabolite adenine arabinonucleoside triphosphate (see Huber et al, Proc. Natl. Acad. Sci U.S.A., 88, 8039-8043 (1991) and EP-A-0 415 731).
For the treatment of hepatic metastases of CRC, it is desirable to control the expression of an enzyme with the transcriptional regulatory sequences of a tumor marker associated with such metastases.
CEA is a tumor associated marker that is regulated at the transcriptional level and is expressed by most CRC tumors but is not expressed in normal liver. CEA is widely used as an important diagnostic tool for postoperative surveillance, chemotherapy efficacy determinations, immunolocalisation and immunotherapy. The TRS of CEA are potentially of value in the selective expression of an enzyme in CEA.sup.+ tumor cells since there appears to be a very low heterogeneity of CEA within metastatic tumors, perhaps because CEA may have an important functional role in metastasis.
The cloning of the CEA gene has been reported and the promoter localised to a region of 424 nucleotides upstream from the translational start (Schrewe et al, Mol. Cell. Biol., 10, 2738-2748 (1990) but the full TRS was not identified.